Hydrostatically steered vehicles have been available for many years and their application to windrowers has been significant. There has been a limit to the transport speed of these windrowers due to instability at high speed with the drive wheels ahead of the castor wheels (cab forward). Higher speeds on the road allow the user to cover large distances without the use of trailers and tow vehicles.
In co-pending application filed Apr. 28, 2005, Ser. No: 11/116,418 and Canadian application filed Apr. 27, 2005, Ser. No: 2,505,458 and assigned to the present assignee is shown a machine in which the machine travels in transport mode with the castor wheels leading the drive wheels (engine forward), and in this position the machine is much more stable and higher speeds are possible.
While in a turn with a self propelled machine that has the steered wheels leading, the centrifugal force at the center of gravity is such that it tends to assist the rear driving wheels in straightening out the machine. On the other hand when in a turn with a self propelled machine that has the driving wheels leading, the centrifugal force at the center of gravity is such that it encourages the turn, and the front driving wheel must overcome this force in order to straighten out the machine. Thus the machine is more stable, or wants to travel more in a straight line, when the castor wheels are leading and the driven wheel are trailing.
Other bi-directional vehicles such as the Ford TV140, although hydrostatically driven, are not hydrostatically steered. The TV140 machine relies on the pivoting action between front and rear sections of the tractor to steer. Control of the TV140 machine is done using hydraulic valving and cylinders.
Coop Implements manufactured the model 742 windrower tractor which is bi-directional. This was done to allow the header to be detached and connected at the drive wheel end of the tractor. This also provided weight to the drive wheels to make the machine more controllable during transport.
In a typical machine of this type, the ground speed and steering of the machine is achieved by varying the flow output from two individual hydraulic pumps each driving a hydraulic motor attached to the drive wheels. In an application using hydraulic piston pumps, this flow is varied by changing the angle of the cam-plate or swash plate inside the pump which varies the displacement of the pump.
To vary the speed of the machine, both pump displacements are changed simultaneously. To steer the machine, the displacement of each pump is varied relative to each other so that the flow to each wheel is different which causes the machine to turn.
To achieve maximum efficiency, it is desirable to set the pumps at the highest possible displacement for any given speed. To obtain maximum speed of the machine, it is therefore desirable to have the displacement of both pumps at maximum when the machine is a full speed. The problem with this is that in order to steer the machine at full speed (with both pumps at full displacement) the steering mechanism must be allowed to decrease the displacement of one of the pumps against the pressure of the speed control tending to maintain it in the maximum position.
In a speed control system without a servo control, when the operator makes a turn at full speed, the steering system forces the ground speed lever back away from the full speed position since it is only retained there by a friction device.
In a servo controlled system, a servo control valve/cylinder is used to control the pump displacement with a smaller force being required by the operator. In this case, the servo force can be quite high in order to resist the pump cam-plate forces in all cases. This can cause a high steering effort required by the operator.